A mass spectrometer typically includes a number of components arranged in-line between an ion source and an ion detector. To ensure a wide range of ions can be efficiently transmitted through the instrument, suitable voltages may be applied to focus ions along the axis and/or towards the exit of these components.
For certain applications it is necessary to introduce relatively large potential drops across or between different regions of the instrument. For instance, ions may be accelerated into a gas-filled collision cell to perform collision induced dissociation (“CID”) by introducing a potential drop at the entrance to the collision cell. This potential drop determines the collision or fragmentation energy.
When operated in such a fragmentation mode, in order to transmit a continuous beam of ions through the instrument, it is necessary for all of the components upstream of the collision cell to float or track the potential drop i.e. collision energy. The total potential drop along the length of the instrument must therefore increase by an amount corresponding to the collision energy.
The same is true when transmitting a continuous beam of ions through any device requiring a large potential drop. For instance, in a drift tube ion mobility separation device ions are caused to separate according to their ion mobility along a DC potential gradient. The components upstream and downstream of the drift tube must track the DC potential gradient. Large potential drops may also be required in the ion source or transfer regions to transmit ions of high mass to charge ratio or to aid desolvation.
In some instrument geometries there may be many upstream devices, which may themselves each require an associated potential drop. Since each component must be raised to at least the same potential as the component disposed adjacently downstream of it, there is a cumulative voltage increase in the upstream direction. The cumulative effect of the various focusing voltages and potential drops results in upstream components being held at relatively high absolute potentials. This can lead to potential electrical breakdown issues.
Large potential drops across an instrument can also lead to other problems such as power supply range, safety, voltage accuracy issues and instrument control complexity.
It is known to enhance the separation characteristics of an ion mobility device using combinations of travelling waves and a reverse axial DC gradient as disclosed, for example, in GB-2409764 (Micromass), GB-2392304 (Micromass) and US 2013/0299690 (Shvartsburg).
Other electrostatic manipulations of ions within an ion guide are described in EP-1271611 (Micromass), GB-2382920 (Micromass) and WO2012/150351 (Berdnikov).
It is desired to alleviate such problems associated with introducing a large potential difference within a continuous beam mass spectrometer.